In recent years, fuel cells have occupied an important position as novel type clean energy sources. And solid polymer electrolytes comprising solid polymer electrolyte membranes having high proton conductivity have been developed for retaining the characteristics of their high output capacity and high energy density, and for capability in miniaturization and weight saving. As the solid polymer electrolyte membranes, hydrated membranes such as a sulfonated polyfluoroolefin (trade name: Nafion, manufactured by E. I. du Pont de Nemours and Company) and acid-doped polybenzimidazole (PBI) membranes are generally known. When methanol is used as a fuel for operating fuel cells, the solid polymer electrolytes are required to have barrier properties to fuel methanol (low methanol permeability) However, the hydrated membranes such as Nafion have a limitation to methanol barrier properties due to occurrence of hydrated proton hopping. On the other hand, the acid-doped PBI membranes are homogeneous membranes. And it is considered that, in the acid-doped PBI membranes, proton hopping occurs through acids forming complexes with basic N--H groups in a base polymer, PBI. Accordingly, in the acid-doped PBI membranes, the proton hopping does not occur by the movement of water. The acid-doped PBI membranes have therefore been expected as the solid polymer electrolytes excellent in methanol barrier properties.
As acid-doped PBI membranes, for example, phosphoric acid-doped PBI membranes were prepared by immersing PBI membranes in phosphoric acid solutions [J. S. Wainright etal., J. Electrochem. Soc., Vol. 142, No. 7, p122, July (1995)]. Acid-doped PBI membranes were obtained by allowing acids to be adsorbed by PBI membranes in aqueous solutions of phosphoric acid or sulfuric acid (U.S. Pat. No. 5,525,436). And acid-impregnated PBI and acid-impregnated alkyl or arylsulfonated PBI membranes, or alkyl or arylsulfonated PBI membranes (Japanese Unexamined Patent Publication No. 9-73908) are proposed, and obtained phosphoric acid-doped PBI membranes show superior characteristics.
However, studies of these phosphoric acid-doped PBI membranes have revealed the following problems.
PBI has slight water absorbing capability, however phosphoric acid has extremely high affinity for water. Therefore, a phosphoric acid-doped PBI is liable to cause wrinkles by water absorption. Accordingly, when an MEA (a membrane electrode assembly in which a membrane and electrodes are assembled) is fabricated using the phosphoric acid-doped (wrinkled) PBI membrane and a stack is assembled, followed by operation of it, the use of the phosphoric acid-doped PBI membrane causes the leakage of gas and liquid. Further, there is a limitation of thin film formation of the phosphoric acid-doped PBI membranes.
In particular, when PBI doped with phosphoric acid at a rate of two or more molecules of per basic imidazole ring constituting PBI (one or more molecules of phosphoric acid per N--H group) is hot pressed in preparing the MEA, free phosphoric acid, not participating in bonding, seeps into an electrode layer or a diffusion layer. In a hydrogen fuel cell, therefore, phosphoric acid which has seeped out also acts as an ionomer. However, when the amount of the seeped phosphoric acid is excessive, diffusion of a reaction gas into a catalytic metal is inhibited.
The phosphoric acid that has seeped into the electrode by the above-mentioned hot pressing is not fixed, so that it is apt to seep out of the electrode when water in a gas reaction cell is condensed by interruption of the operation. Further, when the PBI membrane is immersed in condensed water in the gas reaction cell, or when the PBI membrane is immersed in water and methanol in a liquid supply direct methanol fuel cell (DMFC), phosphoric acid fixed into the PBI membrane is also easily dedoped to run out, resulting in a reduction of ion conductivity of the PBI membrane.
Inorganic phosphoric acid is a strong acid and has extremely high methanol solubility. Therefore, the PBI membranes are conventionally doped with phosphoric acid by immersing the PBI membranes having the basicity (N--H groups) in high concentrated methanol solutions of inorganic phosphoric acid. On the other hand, compounds other than inorganic phosphoric acid (for example, organic phosphoric acid compounds) are low in solubility. Accordingly, high concentrated solutions thereof can not be prepared, the acid dissociation degree of the prepared solutions is also low, and the molecular size of dopants is also large. It is therefore difficult to conduct doping by the above-mentioned immersing method.
As described above, it has been difficult to use the phosphoric acid-doped PBI membranes as the solid polymer electrolyte membranes of the liquid supply DMFCs.